I was able to finally balance the Sony 16-35 GM 2.8 with the Zhiyun Webill Lab gimbal without adding any additional weight. I posted some photos below showing my settings . For this example, I used a Sony a7r3.
I installed the mounting plate backwards, so the heavy side works like a counterweight. I also had to remove the eyepiece to make it work. Adjusting the focal length rubs against the plate, and if this bothers you, you can sand off some of the rubber around the middle of the mounting plate.
The video below shows that it works in all modes including vortex mode.